1. Field of the Invention
The present invention relates to a motor controller for correcting torque ripple caused by a motor or a motor controller and generating smooth torque.
2. Description of Related Art
To control the velocity of a motor having torque ripple, a motor controller can reduce velocity fluctuation resulting from the torque ripple by increasing a velocity loop gain. The velocity fluctuation, however, may not be reliably reduced when the velocity loop gain is low, which has to be addressed.
For example, Japanese Unexamined Patent Application Publication No. 2003-88159 discloses a torque ripple correction apparatus for a synchronous AC motor. The apparatus generates a sine wave signal in accordance with an electrical angle of the synchronous AC motor, and adds the sine wave signal to a torque instruction, as a correction value Tcmp. In particular, the motor is operated at a constant velocity, the torque instruction and electrical angle of the motor are measured and accumulated in a buffer for a given period, the accumulated data is Fourier-transformed to extract first- to m-th-order components for the electrical angle, the extracted data is inverse-Fourier-transformed to generate a sine wave correction signal, and a correction unit adds the generated signal to the torque instruction, thereby correcting the torque ripple.
For another example, Japanese Unexamined Patent Application Publication No. 2001-352791 controls a synchronous motor using phase current, the phase current being obtained by multiplying sine wave current by a current correction coefficient K, the current correction coefficient K being calculated as the sum of even harmonics of the number of phases of current control frequency of the synchronous motor. A coefficient of each even harmonics term in an operation expression of the current correction coefficient K is obtained by inverting a polarity with a rate corresponding to the average torque of frequency components calculated by Fourier-transforming a torque fluctuation waveform when the synchronous motor is controlled with the sine wave current. The phase of a current value I of each of the even harmonics is obtained as the phase of frequency component calculated by Fourier-transforming the torque fluctuation waveform. The torque fluctuation can be reduced by using the current correction coefficient K because the torque fluctuation component in the even harmonics of the number of phases appears greatly by the Fourier transform of the torque fluctuation when control is performed by the sine wave current.